1. Field of the Invention
The present invention relates to operational amplifiers (op-amps) and, more particularly, to low distortion operational amplifiers operating from a low voltage power supply.
2. Description of the Related Art
Audio chips presently enable personal computers (PCs), compact disk (CD) players, and other portable audio devices to execute high quality audio applications. Typical audio chips comprise digital circuitry which occupies approximately 75-80% of the audio chip's silicon space and analog circuitry which occupies the remaining 20-25%. Typically, the analog circuitry comprises an analog-to-digital (A/D) converter, digital-to-analog (D/A) converter and some output amplifiers. Its main function is to convert an analog input into a digital format suitable for the digital circuitry to process, then convert the digital signals back into an analog format suitable to drive an output device, such as a speaker. The digital circuitry occupies the majority of the silicon area and typically performs digital signal processing, such as filtering, noise shaping, and synthesizing, on the converted analog signals. The main function of these audio chips is to implement an entire audio system on one piece of silicon.
The conventional analog circuitry utilized to implement high quality audio chips comprises an op-amp having: 1) two gain stages; 2) a highly linear Miller compensation capacitor positioned between the two gain stages; and 3) an output stage connected to the output of the second gain stage. The output stage loads (i.e., draws excessive current from) the second gain stage and, therefore, two gain stages are required to achieve acceptable gain from the op-amp. The Miller frequency compensation capacitor must be connected between the two gain stages to ensure that the op-amp receives sufficient frequency compensation to produce a linear output having an acceptable level of harmonic distortion.
Unfortunately, the position of the Miller compensation capacitor between the two gain stages precludes its implementation using a metal-oxide-silicon field-effect transistor (i.e., MOSFET) connected as a capacitor (i.e., a MOSFET gate capacitor). That is, implementing the Miller compensation capacitor with a MOSFET gate capacitor is undesirable because its connection between the gain stages would cause it to forward bias as its gate voltage approached and exceeded the threshold voltage, resulting in the capacitance of the MOSFET gate capacitor becoming highly non-linear. As the capacitance of the MOSFET gate capacitor becomes increasingly non-linear, the op-amp's gain becomes non-linear and, therefore, its output produces a total harmonic distortion between -70 to -75 dB. The level of total harmonic distortion required for CD quality applications is less than -96 dB, thus rendering the Miller MOSFET compensation scheme unacceptable.
Accordingly, a highly linear plate capacitor must implement the Miller compensation capacitor. Either a digital or analog fabrication process may be used to fabricate the plate capacitor, but each has a particular disadvantage. The digital fabrication process (i.e., a single-poly process) could be used to form a thick oxide plate capacitor, but that process requires large amounts of silicon to implement. Because the thick oxide plate capacitor consumes large amounts of the audio chip's silicon, the digital fabrication process is not typically used.
The analog fabrication process may be used to form a thin oxide plate capacitor, but the process requires costly additional steps, including the addition of a second layer of polysilicon with a layer of silicon dioxide interposed to form the thin oxide plate capacitor. Even though this fabrication process is very expensive, it is nevertheless conventionally used because, as previously mentioned, a highly linear compensation capacitor must be connected between the two gain stages.
Thus, considering the limited amount of space on the audio chip for the op-amp, the large silicon requirements of the thick oxide plate capacitor or the additional costly layer requirements of the thin oxide plate capacitor provide an inadequate solution in the implementation of high quality audio chips.
Due to the aforementioned problems associated with presently available audio chips, a demand exists for a low cost, high quality op-amp which may be fabricated entirely using a digital fabrication process that does not significantly impact chip size or the amount of silicon required for fabrication. Furthermore, the op-amp must be capable of operating from a 3.3 volt power supply and provide a very low total harmonic distortion such that its output approximates the output quality of compact disks. To accomplish this, the op-amp must provide adequate amplification of audio signals and, at the same time, generate no more than -96 db total harmonic distortion. Accordingly, amplification must be independent of amplitude and frequency (i.e. highly linear) to preserve the integrity of the input signal. This op-amp represents a significant improvement because it furnishes both a cost and size effective audio amplifier for use in all high quality audio applications, including battery powered or portable communication and computer products.